1. Field of the Invention
This invention relates to a process for the polymerization of hexafluoropropene oxide (to be abbreviated as HFPO, hereinafter) and more particularly, to a process for the polymerization of HFPO for producing a difunctional HFPO polymer having a high degree of polymerization.
2. Prior Art
Several processes are known in the art for the polymerization of HFPO. For example, U.S. Pat. No. 3,250,807 discloses that difunctional HFPO polymers are prepared by reacting HFPO with FOC--(CF.sub.2).sub.n --COF wherein n is from 0 to 6, in an aprotic polar solvent in the presence of a catalyst such as an alkali metal fluoride (represented by MF) or activated carbon according to the following reaction scheme. ##STR1##
Such an attempt to add HFPO to the previously furnished polymerization initiator (FOC--(CF.sub.2).sub.n --COF in the above process) often gives rise to the problem that a HFPO homopolymer is formed as a by-product due to chain transfer reaction as shown below. ##STR2## polymerization activating terminal Separation of MF from polymerization activating terminal ##STR3## Ring opening (or chain transfer) of HFPO by MF ##STR4## Formation of HFPO homopolymer ##STR5##
One solution to the above problem is disclosed in JP-B 5360/1978 and USP 3,660,315. According to this process, a difunctional polymer of formula (2a) is obtained using a tetraglyme solution of a compound of formula (1a) as the polymerization initiator. ##STR6##
This process is characterized by mixing cesium fluoride with FOCCF(CF.sub.3)OCF.sub.2 CF.sub.2 OCF(CF.sub.3)COF in tetraglyme, and separating the excess of cesium fluoride from the solution to give a homogeneous solution, and effecting polymerization of HFPO in the homogeneous solution. This prevents the homo-polymerization of HFPO which is caused by the excess of cesium fluoride, and therefore, suppresses the formation of a monofunctional (or one-end functional) HFPO polymer.
U.S. Pat. No. 4,356,291 describes that high molecular weight HFPO polymers are obtainable using highly purified HFPO because HFPO generally contains impurities such as hydrogen fluoride, acid fluorides and water which undesirably restrict the maximum degree of polymerization of polymers resulting from polymerization of HFPO. Also, J. Macromol. Sci. Chem., A8 (3), 499-520 (1974) describes that the presence of hexafluoropropene (to be abbreviated as HFP, hereinafter) during HFPO polymerization is effective for preventing chain transfer reaction and therefore, increasing the degree of polymerization of the resulting polymer. The alleged effect of HFP is to trap free cesium fluoride.
It is also known that the addition of water or alcohols to the HFPO polymerization system promotes chain transfer reaction so that only oligomers having a low degree of polymerization are produced. These protonic compounds form hydrofluoric acid with active terminals available in the HFPO polymerization system in accordance with the reaction scheme shown below, and hydrofluoric acid functions as a chain transfer agent. ##STR7##
Therefore, in the prior art, the method of preventing chain transfer reaction during HFPO polymerization for increasing the degree of polymerization of HFPO polymers has been investigated from the aspect of removing free cesium fluoride in the HFPO polymerization system and the chain transfer reaction-inducing substances in the HFPO reactant.
The known polymerization initiator for use in HFPO polymerization is generally prepared by mixing and agitating a perfluorodicarboxylic acid fluoride represented by the formula (3): EQU FOC--Rf--COF (3)
wherein Rf is a perfluoroalkylene group of 1 to 4 carbon atoms or perfluoroalkylene group of 2 to 10 carbon atoms having an ether bond, with cesium fluoride in an aprotic polar solvent, followed by precipitation and separation of the excess of cesium fluoride. One reactant, perfluorodicarboxylic acid fluoride is highly hydrolyzable while the other reactant, cesium fluoride is highly hygroscopic. Therefore, the series of steps must be carried out through a vacuum line or in a glove box, which operation is cumbersome. Even the carefully prepared solution has a possibility of containing water, hydrogen fluoride and other impurities that can induce chain transfer reaction.
Accordingly, there is a desire to have a process for producing a HFPO polymer that can minimize the above-mentioned possibility and ensures to simply prevent chain transfer reaction.